Calcium aluminate cement based catalyst

ABSTRACT

Shaped particles suitable for use as a catalyst, or precursor thereto, particularly for the decomposition of hypohalite ions in aqueous solution, comprising a high alumina cement having an aluminium to calcium atomic ratio above 2.5 and at least one oxide of a Group VIII metal M selected from nickel and cobalt, said particles containing 10 to 70% by weight of said Group VIII metal oxide and having a porosity in the range of 30 to 60%, in which at least 10% of the pore volume is in the form of pores of size in the range 15 to 35 nm and less than 65% of the pore volume is in the form of pores of diameter greater then 35 nm are disclosed.

This application is the National Stage of International No.PCT/GB96/01698 filed Jul. 16, 1996.

This invention relates to catalysts and in particular to catalysts, orprecursors thereto, containing an inert support material and at leastone oxide of a metal of Group VIII of the Periodic Table and selectedfrom nickel and cobalt.

In our EP 0 397 342 we describe catalysts in the form of shapedparticles, e.g. extrudates, containing a calcium aluminate cement, anoxide of cobalt and/or nickel, and optionally a finely divided diluentmaterial, the shaped particles having specified porosity and specifiedpore size distribution characteristics. These catalysts were ofparticular utility for the decomposition of hypochlorite ions in anaqueous medium.

In that specification we indicated that increasing the porosity of theparticles was desirable as it allows the reactants to have ready accessto the active material within the particles. However this had thedisadvantage that the strength of the particles was decreased.

We have now found that by employing a calcium aluminate cement having ahigh alumina content, the porosity can be increased without undue lossof strength and as a result the activity of the catalyst can beincreased. Surprisingly, despite the higher porosity, the bulk densityof the particles can be increased so that a greater mass of particles,and hence active material, can be accommodated in a catalyst bed ofgiven volume. [The bulk density is determined by filling a vessel ofknown volume with the catalyst particles, with tapping of the vessel toensure that the particles settle, and then determining the weight ofparticles in the vessel.]

Calcium aluminate cements are hydraulic cements containing one or morecalcium aluminate compounds of the formula nCaO.mAl₂ O₃ where n and mare integers. Calcium aluminate compounds mentioned in the aforesaidspecification include calcium mono-aluminate CaO.Al.₂ O₃, tri-calciumaluminate 3CaO.Al₂ O₃, penta-calcium tri-aluminate 5CaO.3Al₂ O₃,tri-calcium penta-aluminate 3CaO.5Al₂ O₃, and dodeca-calciumhepta-aluminate 12CaO.7Al₂ O₃. The calcium aluminate cement used in theexamples of that specification had an aluminium to calcium atomic ratioof about 1.4. By the term high alumina cement, we mean a calciumaluminate cement having an aluminium to calcium atomic ratio above 2.5.Such cements are known and may contain one or more of the above calciumaluminate compounds, and or compounds such as calcium di-aluminateCaO.2Al₂ O₃, in some cases with additional alumina.

Accordingly the present invention provides shaped particles suitable foruse as a catalyst, or precursor thereto, comprising a calcium aluminatecement having an aluminium to calcium atomic ratio above 2.5, andpreferably above 4.0, and at least one oxide of a Group VIII metal Mselected from nickel and cobalt, said particles containing 10 to 70% byweight of said Group VIII metal oxide (expressed as the divalent oxide,MO) and having a porosity in the range 30 to 60%, particularly 40 to55%, in which at least 10% of the pore volume is in the form of pores ofsize in the range 15 to 35 nm and less than 65% of the pore volume is inthe form of pores of diameter greater than 35 nm.

As described in the aforesaid EP 0 397 342, for some catalyticapplications the Group VIII metal oxide is the catalytically activespecies while for other catalytic applications the Group VIII metaloxide is a catalyst precursor and the catalytically active species isthe product of reducing the Group VIII metal oxide to the Group VIIImetal or is the product of oxidising the Group VIII metal oxide in theprecursor to a higher oxidation state. For example catalysts obtained byreduction of a precursor containing nickel and/or cobalt oxide are ofuse as hydrogenation catalysts, e.g. methanation catalysts for thehydrogenation of carbon oxides to methane or catalysts for thehydrogenation of aromatic compounds such as benzene to cyclohexane.Another use of supported nickel and/or cobalt oxides is as catalysts forthe decomposition of oxidising agents such as hypochlorite ions inaqueous solutions, for example in the treatment of effluents containingsuch ions prior to discharge of into rivers, lakes, or estuaries.

The shaped particles are preferably in the form of granules, extrudates,or pellets and preferably have an aspect ratio, by which we mean theratio of the weight average maximum geometric dimension, e.g. length, toweight average minimum geometric dimension, e.g. diameter, of less than3, particularly less than 2. Particles having a greater aspect ratio maybe liable to suffer from breakage during use. The shaped particlespreferably have a weight average maximum dimension in the range 2 to 8mm, particularly 3 to 8 mm. This ensures that the particles have arelatively high a geometric surface area per unit bed volume, so that abed of the particles has a relatively large external particle areaexposed to the reactants without the presence of an undue proportion offines which would lead to unacceptable pressure drop on passage ofreactants through a bed of the particles.

The particles of the invention have a porosity in the range 30 to 60%,particularly 40-55%. By the term porosity we mean the ratio of thevolume of the pores to the volume of the particle. Porosity may bedetermined by measurement of the mercury and helium densities of theparticles: the porosity (as a percentage) is given by

    porosity=P.sub.Hg ×[1/p.sub.Hg -1/p.sub.He ]×100

where P_(Hg) and P_(He) are respectively the densities of the particlesmeasured by displacement of mercury and helium. The mercury density isthus a measure of the particle density, while the helium density is ameasure of the skeletal density.

The particles of the invention have a particular pore size distribution.This may be determined by mercury intrusion porosimetry. In theparticles of the invention, at least 10%, and preferably 10 to 40%, ofthe pore volume is in the form of pores of average diameter in the range15-35 nm and less than 65% of the pore volume is in the form of pores ofaverage diameter above 35 nm. Particles having such a pore size are ofparticular utility where they are used for the decomposition ofoxidising agents in aqueous media.

Largely as a result of the porosity and pore size distribution, theparticles also have a relatively high BET surface area, above 10, and inparticular in the range 20-100, m².g⁻¹. As a result the active materialis present in a finely divided state. Such a BET surface area may beachieved by introducing the Group VIII metal oxide into the compositionby a precipitation route as described hereinafter.

As a result of their composition and porosity, the shaped particles ofthe invention have a bulk density in the range 0.8 to 1.5, preferably0.9 to 1.4, g.cm⁻³. The bulk density is indicative of the weight ofcatalyst in a bed of given volume.

During use of the particles as a catalyst for the decomposition ofoxidising agents, e.g. in effluents, the BET surface area, porosityand/or pore size distribution may change: thus the BET surface area,porosity, and the proportion of pores of size less than 35 nm mayincrease. The surface area, density, and porosity parameters of theshaped particles referred to herein refer to the parameters of theparticles in the "as made" state, i.e. before use for catalyticpurposes.

Shaped particles having the required porosity and pore volumecharacteristics may be made by a particular pelleting method asdescribed hereinafter.

In addition to the high alumina cement, the composition comprises atleast one oxide of a Group VIII metal selected from nickel and cobalt.Preferably the Group VIII metal is nickel alone, or nickel in admixturewith cobalt in an amount of up to one mole of cobalt per mole of nickel.

Calcium aluminate cements are often contaminated with iron compounds,e.g. iron oxide. In the aforesaid EP 0 397 342 we indicated that thepresence of iron oxide was beneficial where the shaped particles were tobe used for hypochlorite decomposition as the iron exhibited somepromoting effect on the catalytic activity. In contrast, in the presentinvention we have found that high activity catalysts can be producedwith high alumina cements of low iron oxide content. In the presentinvention, the iron oxide content (expressed as Fe₂ O₃) of the shapedparticles is preferably less than 1% by weight. A particularly suitablehigh alumina cement having a low iron oxide content is that known asCA-25 which typically contains about 80% alumina, primarily asmono-calcium aluminate in admixture with dodeca-calcium hepta-aluminate,calcium di-aluminate, and free alumina.

The Group VIII metal oxide is preferably introduced into the compositionby precipitation. A preferred route is to precipitate Group VIII metalcompounds, decomposable to oxides by heating, from an aqueous solutionof e.g. nitrates by addition of a precipitant such as an alkali metalcarbonate solution. After precipitation of the Group VIII metalcompounds, the precipitate is washed free of precipitant. Theprecipitate may be mixed with a finely divided, preferably inert,diluent material, such as magnesia and/or a clay, e.g. kaolin. Theamount of such diluent material employed is conveniently up to twice theweight of the Group VIII metal compounds expressed as the divalentoxides. The mixture is then dried, and calcined, e.g. to a temperaturein the range 200-600° C., particularly 400-550° C., to effectdecomposition of the Group VIII metal compounds to the oxide form. Minoramounts of other ingredients, such as co-promoters such as magnesiumoxide may be incorporated, e.g. by co-precipitation with the Group VIIImetal compounds. The resultant composition is then mixed with the highalumina cement, optionally together with a processing aid such as alittle water, a stearate of an alkaline earth metal, e.g. magnesium,and/or graphite, and formed into pellets. The proportion of cementemployed is generally 25 to 100% by weight based on the total weight ofthe Group VIII metal oxide, or oxides, and any diluent material, and issuch as to give a composition containing 10 to 70%, particularly lessthan 50%, and most preferably 20 to 40%, by weight of the Group VIIImetal oxide or oxides.

In order to obtain shaped particles of the requisite pore volumecharacteristics, the mixture is conveniently pelletised by means of apellet mill, for example of the type used for pelleting animalfeedstuffs, wherein the mixture to be pelleted is charged to a rotatingperforate cylinder through the perforations of which the mixture isforced by a bar or roller within the cylinder. The resulting extrudedmixture is cut from the surface of the rotating cylinder by a doctorknife positioned to give pellets of the desired length. It will beappreciated that other extrusion techniques may be employed to giveshaped particles of the desired characteristics.

After forming the composition into the desired shaped particles, thelatter are then preferably contacted with water, preferably as steam, toeffect hydration of the cement and to give the shaped particles adequatestrength.

Shaped particles formed by this method have a significantly lowerstrength, e.g. as measured by a crushing test, than pellets prepared bya conventional tabletting technique but it is found that, even so, thestrength is adequate for the applications envisaged and, indeed, thestrength generally increases where the catalyst is employed for thedecomposition of oxidising agents in aqueous media, presumably as aresult of continued hydration of the cement.

For use for the decomposition of oxidising agents, the catalyst bed iscontacted with a fluid medium, particularly aqueous, containing theoxidising agent to be treated. Examples of oxidising agents that may bedecomposed using the shaped particles of the invention includehypohalite ions, for example hypochlorite and hypobromite ions, andhydrogen peroxide. At least some of such oxidising agents are pollutantsin various industrial processes. In particular hypochlorite ions are asignificant industrial pollutant. The catalysts may also find utility inthe treatment of aqueous media containing organic pollutants: thus ashypochlorite may be added to an aqueous medium containing an oxidisableorganic compound and the solution passed through a bed of the catalyst.The catalyst catalyses the decomposition of the hypochlorite whicheffects oxidation of the organic compound to more environmentallyacceptable products such as carbon dioxide and water.

Conveniently a fixed bed of the catalyst particles is formed and themedium containing the oxidising agent, for example hypochlorite ions, ispassed through the bed. Generally the medium is in the form of anaqueous solution which has been filtered prior to contact with thecatalyst bed.

The treatment of aqueous media is conveniently effected under conditionssuch that the pH of the medium is above 7, preferably above 8; it is aparticularly beneficial aspect of the invention that the particles donot physically disintegrate even at pH levels in the range 10 to 14. Theprocess can be performed at any convenient temperature, suitably in therange 5-100° C., more suitably in the range 20-80° C.

When the shaped particles are contacted with the oxidising agent in anaqueous medium, some or all of the oxides of the particles may becomehydrated. In addition the Group VIII metal oxides are oxidised to highervalency states. For example nickel oxide can be notionally considered tobe initially present in the particles as NiO. Authorities vary as toprecisely what higher oxides of nickel are formed but it may be regardedthat the higher oxides Ni₃ O₄, Ni₂ O₃ and NiO₂ are formed on contactwith the oxidising agent. Such higher oxides are active in the processof decomposition of the oxidising agent. In the particles of the presentinvention, the Group VIII metal oxides may be as initially formed or intheir higher oxidation states, as formed in use. In use the oxides mayalso be present as hydrates. It should be noted, however, that theproportions specified herein of the Group VIII metal oxide in theparticles are expressed on the basis of anhydrous oxides with the GroupVIII oxides in the divalent state, i.e. NiO and/or CoO.

In addition to use for the decomposition of oxidising agents asdescribed above, the shaped particles of the invention are also of useas precursors to hydrogenation catalysts, and may be converted to thecatalytically active form by reduction, e.g. with a stream of ahydrogen-containing gas at an elevated temperature. Such reduction maybe effected after charging the particles to a vessel in which thehydrogenation is to be effected. Alternatively, the reduction may beeffected as a separate step prior to charging the particles to thehydrogenation reactor and, if desired, the reduced particles may bepassivated by contact with a gas stream containing a small amount ofoxygen, or with carbon dioxide followed by a gas stream containing asmall amount of oxygen, until no further reaction occurs when theparticles may then be handled in air at ambient temperature.

The invention is illustrated by the following example in which all partsand percentages are by weight.

A slurry containing precipitated basic nickel carbonate, and a mixtureof finely divided magnesia and kaolin as diluent materials, wasfiltered, washed, dried, and then calcined at 400-450° C. Theproportions of the ingredients were such that the calcined materialcontained about 14.1 parts of magnesia and about 113 parts of kaolin per100 parts of nickel oxide.

100 parts of the calcined material were then mixed with about 2 parts ofgraphite and 41 parts of a high alumina cement, CA-25, having analuminium to calcium atomic ratio of about 4.9 and having an ironcontent of about 0.2% to give a dry feed mixture.

The dry feed mixture was then mixed with water (25 parts per 100 partsof the cement-containing mixture), formed into extruded pellets ofdiameter of about 3 mm and lengths in the range of about 3 to 5 mm usinga pellet mill as described hereinbefore, and then dried to giveextrudates A.

For purposes of comparison the above procedure was repeated using acalcium aluminate cement having an aluminium to calcium atomic ratio ofabout 1.1 and an iron oxide content (expressed as Fe₂ O₃) of about 14%in place of the high alumina cement. The resultant extrudates weretermed extrudates B. The properties of the extrudates are shown in thefollowing table.

    ______________________________________                                                           Extrudates                                                                    A    B                                                     ______________________________________                                        Calculated  NiO          31.3   31.1                                            composition Al.sub.2 O.sub.3 38.5 26.9                                        (wt %) - after CaO 5.3 11.7                                                   ignition at MgO 4.5 4.6                                                       900° C. SiO.sub.2 20.0 21.2                                             Fe.sub.2 O.sub.3 0.4 4.5                                                     Porosity (%)  47 35                                                           Pore volume 15-35 nm 16 35                                                     >35 nm 54 38                                                               Bulk density (g/ml)  1.20   1.05                                                BET surface area (m.sup.2 /g) 46 43                                         ______________________________________                                    

The activity of the catalysts for the decomposition of hypochlorite wasassessed by charging 120 ml of the extrudates to a reactor of internaldiameter 2.5 cm to form a catalyst bed therein. A feed of an aqueoussolution containing 63.1 g/liter of sodium hypochlorite and having a pHof about 12.5 was preheated to about 30° C. was fed to the reactors at aspace velocity of 0.8 h⁻¹ so that the hypochlorite solution flowed upthrough the catalyst bed. The exit sodium hypochlorite 10 concentrationwas found to be 0.56 g/l for extrudates A and 1.85 g/l for extrudates Bindicating that the extrudates had a significantly greater activity. Inaddition, from the above table, it is seen that A had a significantlygreater bulk density than that of extrudates B.

I claim:
 1. Shaped particles suitable for use as a catalyst, orprecursor thereto, comprising a calcium aluminate cement having analuminium to calcium atomic ratio above 4.0 and at least one oxide of aGroup VIII metal M selected from nickel and cobalt, said particlescontaining 10 to 70% by weight of said Group VIII metal oxide (expressedas the divalent oxide, MO) and having a porosity in the range 30 to 60%in which at least 10% of the pore volume is in the form of pores of sizein the range 15 to 35 nm and less than 65% of the pore volume is in theform of pores of diameter greater than 35 nm.
 2. Shaped particlesaccording to claim 1 having a bulk density in the range 0.8 to 1.5g/cm⁻³.
 3. Shaped particles according to claim 1 wherein 10 to 40% ofthe pore volume of the particles is in the form of pores of averagediameter in the range 15 to 35 nm.
 4. Shaped particles according toclaim 1 containing less than 1% by weight of iron oxide (expressed asFe₂ O₃).
 5. Shaped particles according to claim 1 comprising saidcalcium aluminate cement in admixture with i) a finely divided GroupVIII metal M oxide obtained by calcination of a precipitated Group VIIImetal M compound and ii) a finely divided diluent material in an amountof up to twice the weight of said Group VIII metal M oxide.
 6. Shapedparticles according to claim 5 containing 25 to 100% by weight of saidcalcium aluminate cement based on the total weight of said Group VIIImetal M oxide and any diluent material.
 7. Shaped particles according toclaim 1 containing 20 to 40% by weight of said Group VIII metal M oxide.8. A process for the decomposition of hypochlorite ions in an aqueousmedium comprising contacting said medium with a fixed bed of shapedparticles containing a calcium aluminate cement having an aluminum tocalcium atomic ratio above 4.0 and at least one oxide of a Group VIIImetal M selected from nickel and cobalt, said particles containing 10 to70% by weight of said Group VIII metal oxide (expressed as the divalentoxide, MO) and having a porosity in the range 30 to 60% in which atleast 10% of the pore volume is in the form of pores of size in therange 15 to 35 nm and less than 65% of the pore volume is in the form ofpores of diameter greater than 35 mn.
 9. A process according to claim 8wherein the aqueous medium has a pH above 8.